air bladder

from The Century Dictionary.

• noun See air-float.
• noun In entomology, one of the numerous bladder-like endings of smaller tracheæ? in the bodies of many insects, which, filled with air, greatly reduce the specific gravity of the insect.
• noun A vesicle in an organic body filled with air.
• noun In ichthyology, the sound or swim-bladder; a symmetrical bladder or sac filled with air, generally situated directly under the vertebral column in front, and homologous with the lungs of airbreathing animals.

from the GNU version of the Collaborative International Dictionary of English.

• (Anat.) An air sac, sometimes double or variously lobed, in the visceral cavity of many fishes. It originates in the same way as the lungs of air-breathing vertebrates, and in the adult may retain a tubular connection with the pharynx or esophagus.
• A sac or bladder full of air in an animal or plant; also an air hole in a casting.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An internal organ that fish use to control their buoyancy, allowing them to maintain or change depth by changing their density.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an air-filled sac near the spinal column in many fishes that helps maintain buoyancy